In the early days of telecommunication, a copper wire medium was used to carry a single information channel. Because the greatest proportion of cost is in the materials and construction of the physical link, telephony engineers have developed ways to pack multiple channels onto a single physical link. Frequency division multiplexing (FDM) and time division multiplexing (TDM) have been devised to multiplex multiple streams of analog and pulse code modulation (PCM) digital signals, respectively, into one. For digital signals, the time division multiplexing hierarchy is DS0 through DS4, where a DS0 is a single 0.064 Mbps channel and a DS4 is 4,032 message channels (DS0s) multiplexed together.
A similar time division multiplexing scheme is used in international telephone systems based on 32-channel format. The international digital systems, based on International Telecommunication Union CCITT's G.700 Series Recommendations, are commonly called E1 or CEPT-1. The E1 signals are based on blocks of 32 channels or time slots, of which time slot 0 and time slot 16 are typically used for control and signaling, respectively.
In today's telecommunications network, digital cross-connect (DCS) systems typically reside at the central office as part of the telecommunications carrier or transmission network. Digital cross-connect systems are the interconnection points for terminals, multiplexers, and transmission facilities. The digital cross-connect system may be a non-blocking matrix that remaps DS1 signals electronically at the DS0 level. There is an increased interest to broaden the application of the digital cross-connect system to end office operations and to interface directly with E1 signals, for example.